refactor: use in-cluster authentication instead of kubeconfig secret

RATIONALE:
Gitea CI runner is running IN-CLUSTER, so we should use Kubernetes' built-in
in-cluster authentication mechanism instead of storing kubeconfig secrets.

IN-CLUSTER AUTHENTICATION:
- Kubernetes automatically mounts service account token
- Location: /var/run/secrets/kubernetes.io/serviceaccount/token
- Location: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
- kubectl automatically detects and uses these
- No need to pass credentials via secrets

CHANGES:
1. Remove KUBECONFIG_B64 secret requirement
2. Add in-cluster auth detection step
3. Update Job to use actual built image (not golang base)
4. Job uses imagePullSecrets for registry auth (can be encrypted with SOPS)
5. Add regcred image pull secret reference

CI FLOW:
  1. Detect in-cluster authentication is available
  2. kubectl commands automatically use mounted service account
  3. No secrets needed in CI env vars
  4. Job applies with RBAC service account
  5. Registry credentials via imagePullSecrets (encrypted with SOPS)

SECURITY:
✓ In-cluster auth is more secure (bound to service account)
✓ No kubeconfig stored in secrets
✓ Sensitive data encrypted with SOPS
✓ Principle of least privilege (service account RBAC)

k8s/integration-test-job.yaml

@@ -15,27 +15,25 @@ spec:
       restartPolicy: Never
       containers:
       - name: integration-tester
-        image: golang:1.26-bookworm
-        imagePullPolicy: IfNotPresent
-        workingDir: /workspace
+        image: forgejo.riotpiao.com/rock/api-gateway:latest
+        imagePullPolicy: Always
+        workingDir: /app
         command:
-          - /bin/bash
+          - /bin/sh
           - -c
           - |
             set -e
             echo "Starting integration tests..."
+            echo "Gateway URL: http://api-gateway:8080"
             
-            # Clone the repo
-            git clone https://forgejo.riotpiao.com/riotpiao-poimen/homelab-frontend.git .
-            
-            # Wait for gateway to be ready
+            # Wait for gateway service to be ready
             echo "Waiting for gateway service to be ready..."
-            for i in {1..30}; do
-              if curl -s http://api-gateway:8080/healthz | grep -q "alive"; then
+            for i in $(seq 1 30); do
+              if curl -s http://api-gateway:8080/healthz > /dev/null 2>&1; then
                 echo "✓ Gateway is ready"
                 break
               fi
-              echo "Attempting to reach gateway ($i/30)..."
+              echo "Waiting for gateway... ($i/30)"
               sleep 2
             done
             
@@ -47,6 +45,8 @@ spec:
         env:
         - name: GATEWAY_URL
           value: "http://api-gateway:8080"
+        - name: CI
+          value: "true"
         resources:
           requests:
             cpu: 250m
@@ -72,4 +72,6 @@ spec:
         emptyDir: {}
       - name: home
         emptyDir: {}
+      imagePullSecrets:
+      - name: regcred
   backoffLimit: 1